We have poposed three lines of work to help understand the molecular mechanisms of induced mutation in mammalian cells: 1) Determination of the role of base sequence and of combination with protein on the reactivity of DNA with mutagenic agents; 3) Determination of the biochemical mechanisms for the excision repair of chemically induced damage by both in vivo studies: and 3) Study of the mechanisms by which mammalian cells can replicate past lesions in their DNA. In the coming year we propose to approach these objectives by the following series of experiments: A) Use of phi X174 DNA damaged by reaction with acetoxyacetylaminofluorence and other electrophilic reagents to determine how interaction with different DNA polymerases and metal ions determine the extract nucleotide site at which DNA systhesis is blocked. We want to examine the role of the 3'-5 exonuclease in establishing the site of inhibition of synthesis. B) Purification of the apurinic/apyrimidinic nuclease with associated 5'-3' exonuclease activity from lymphoma cells to determine whether both activities are on a single protein molecule and for use of the enzyme(s) as a reagent for the study of mutagen-DNA interactions. We plan to study the behavior of the multi-enzyme systems resulting from combination of this nuclease with bacterial and mammalian DNA polymerase. C) In vivo studies using excision defective xeroderma pigmentosum lymphoblastoid lines to understand the biosynthetic bypass of lesion in DNA by measurements of the amount of single strandedness in DNA isolated from benzoylated naphthoylated DEAE cellulose columns.